An undervoltage release (UVR) device is a well known circuit breaker accessory utilized to protect electrical equipment against potential damage from an abnormal undervoltage condition. Typically, a UVR device takes the form of a solenoid energized from the line voltage to magnetically detain its plunger in a seated position against the bias of a calibrated return spring. If line voltage drops to an undesirably low level for longer than a momentary period, the plunger detaining magnetic force decreases to the point where the return spring force becomes overpowering. The plunger thus springs to a retracted position, initiating tripping of the circuit breaker to its open circuit condition. After the undervoltage condition has been corrected, it is necessary to reset the UVR device. This reset operation entails forceably returning the plunger to its seated position in order that the magnetic force generated by the solenoid can regain control of the plunger such as to retain it in its seated position against the return spring bias. One manner of effecting this reset operation is manually by depression of a button to force the plunger back to its seated position. However, the preferred method is to utilize the opening movement of the circuit breaker operating mechanism to return the plunger to its seated position and hold it there against the return spring bias pending the return of normal line voltage. U.S. Pat. Nos. 3,162,740; 3,293,577; 3,360,751 and 3,453,568 are exemplary of this prior art approach to resetting a UVR device.
In the typical circuit breaker, the opening movement of its mechanism is quite violent, necessarily so to achieve the abrupt contact separation required in most applications. In order that the full impact of the breaker mechanism opening movement not be communicated to the UVR device, resilient means, typically in the form of a spring, is generally interposed between the breaker mechanism and the UVR plunger. Such resilient means yields to cushion the force of impact of the plunger upon arrival at its seated position, as well as to accommodate overtravel of the breaker mechanism in its opening movement. Thus, the resilient means also accommodates less strigent manufacturing tolerances.
The principle problem encountered in the field with UVR devices is their failure to reset. If the UVR device cannot be reset, its plunger remains retracted by the return spring, and thus reclosure of the breaker contacts to restore electrical service is inhibited. Reset failures are generally occasioned by the inability of the breaker mechanism acting via the cushioning means to firmly reseat the solenoid plunger and thereby render the solenoid magnetic force capable of detaining the plunger in its seated position against the bias of the return spring. It is found that in time increased frictional drag on the plunger movement caused by wear, dirt accumulation and misalignment can render the cushioning means incapable of reseating the plunger to the extent necessary to enable the magnetic force of the solenoid to overpower the return spring.
It is accordingly an object of the present invention to provide an improved undervoltage release device for utilization as an accessory for electric circuit breakers.
Another object of the present invention is to provide an undervoltage release device of the above character which is automatically reset incident to normal functioning of the breaker operating mechanism.
A further object is to provide an undervoltage release device of the above character which is automatically reset in an efficient and reliable manner.
Yet another object of the present invention is to provide an undervoltage release device of the above character which is uniquely constructed to provide long-lived, trouble-free operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.